1. Field of the Invention
The present invention relates to a microscope optical system, and particularly relates to a microscope optical system provided with an intermediate magnification varying part and a microscope objective lens.
2. Related Background Art
When cells of an organism, for example, are to be observed, a general process is that a stage is moved under a relatively large field of view at a low magnification so that an object to be observed such as a cancer cell is brought into the field of view and then the magnification is changed to a high magnification so that the object can be observed closely. In the case of examination of cancer cells, cytological examination is generally performed, in which cells or blood sampled from a patient is observed by a microscope. In the cytological examination, observation is performed firstly with a relatively large field of view using a microscope objective lens with a magnification of 10× while moving a stage, and if a cell suspected as a cancer cell or the like is discovered, the magnification of the microscope objective lens is switched to 40× for observation with a high resolution. In cytological examination, microscope objective lenses with magnifications of 10× and 40× are typically switched and used.
Such magnification switching is generally performed by switching objective lenses. In the case that objective lenses are switched, an eccentricity in the barrel and a displacement of a focal point position occur in the switching process. Therefore, it is necessary to allow adjustment of those factors by adding an adjusting mechanism in the revolver or by moving the stage.
In addition, in cytological examination, an operation called marking is performed, that is an operation of putting a mark on the cover glass at a position near a suspicious cell.
In order to eliminate need for the above-described adjustment operations or to simplify those operations, a process of changing the observation magnification by operating a magnification varying portion provided on an arm of a microscope has been proposed, for example in Japanese Patent Application Laid-Open No. 2001-166215.
Furthermore, in Japanese Patent Application Laid-Open No. 9-33818, there is proposed to change the magnification by a magnification varying apparatus without switching the microscope objective lens. In that case, it is preferable that a microscope objective lens with a magnification of 20× or so be used. However, the numerical apertures of microscope objective lenses with a magnification of 20× or so are generally equal to or smaller than 0.5.
The methods of varying magnification disclosed in the aforementioned prior arts suffer from a problem that in the case that an enlarging magnification varying system is used in combination with an objective lens with a relatively low magnification, high resolution observation is not possible due to the smallness in the numerical aperture of the objective lens. On the other hand, in the case that a reducing magnification varying system is used in combination with an objective lens with a relatively high magnification, there is another problem that aberrations at the periphery of the field of view is deteriorated. In order to eliminate the aforementioned problems, the optical system arrangement in which an image is once formed by the light from the objective lens and then the image is enlarged by the magnification varying part has been adopted. However, in the case that an intermediate magnification varying apparatus is used in combination with a microscope, the eye point will be made high and the size of the apparatus will be increased, so that fatigue of the observer tends to be increased when observation is performed for a long period of time. In addition, in that case, the distance between the objective lens and the imaging lens becomes large and the light quantity in the image periphery will be decreased, so that there is a risk that vignetting can occur.
In addition, in the case that an objective lens with a high magnification is used, there is another problem that the marking operation is not easy since the working distance becomes small.